The ability of the Defense Mechanism Inventory (DMI) to predict the expression of agression was evaluated by subjecting 34 male studients to an unfair, unpleasant attack for coming late to an experiment. A baseline for each subject had been established in a prior, neutral session where attitudes toward psychological experiments and experimenters had been assessed. The differences in attitudes from the first session to that in which the subject had been the victim of aggression were significantly related to four of the five DMI clusters, as was the observed level of the subject's hostility. A composite DMI score, computed by a linear resolution of the clusters into an acting-out continuum, is suggested as a more comprehensive predictor of the reaction to agression.